Progressive cavity pumps pump a fluid by a staged movement of small sealed cavities that progress from one end of the pump to the other. Progressive cavity pumps are commonly used for oil and gas well recovery or extraction projects where there may not be enough formation back pressure to drive fluids to the surface and a form of artificial lift is required. A progressive cavity pump includes the pump components downhole, an actuator above grade and a rod string to connect the actuator to the downhole pump components. Depending upon how deep the well is the rod string can be quite long and subjected to significant forces. Rod strings are typically made from steel or other durable materials.
On occasion, the rotor of a progressive cavity pump needs to be pulled or lifted from its mating stator for maintenance, repairs, or to free a rotor that gets stuck or jammed within the stator. However, when the upper end of the rod string is disengaged from the drivehead to pull the rotor, there is a tendency for the rotor and rod string to “backspin”. Backspin is a condition which occurs when a well head is being shut down for work over for example or restarted after such a work over. The tendency to backspin results from the combination of two factors. First, the rod string functions like a powerful torsion spring when it is decoupled from the drivehead. The build-up of torsional energy in the very long (relative to its diameter) rod string resulting from the twisting referred to above tends to rotate the rod string backwards. Second, when the rotor is pulled from the stator, the column of fluid (i.e., fluid head) above the progressive cavity pump is no longer supported by the pump and will tend to flow back down under the force of gravity past the pulled rotor and through the stator. As the fluid flows past the rotor it tends to cause the helical-shaped rotor to function like a progressive cavity motor and rotate backwards, thus applying torque to the rod string.
The acceleration and rotational velocity of a back-spinning rod string presents a variety of potential safety hazards at the wellhead on the surface. For instance, the upper end of the rod string, which may for example be in the form of a “polished rod”, can tend to bend over while back-spinning, potentially impacting nearby persons or objects. In addition, a bent polished rod may send debris flying across the worksite. Further, extreme vibrations generated by the violent back-spinning may cause weaken or damage the support structure surrounding the rod string at the wellhead. Moreover, in some cases, contact between metal parts with high relative rotational velocities may result in sparks that could ignite combustible gases and hydrocarbon liquids at the surface.
Backspin retarders have been developed to control backspin until the fluid head and sucker rod wind-up have been reduced to safe levels. Existing backspin retarders have met with varying degrees of success. Typical prior art of retarders use built-in pumps to discharge fluid through an orifice and cause increased braking as speed increases. Some retarders have been placed on drive motor shafts and rely on drive belts. However, drive belts have been known to break, in which case backspin protection is lost completely.
Prior art patents of general interest in the field of backspin retarders include: U.S. Pat. No. 4,714,110 (Dysarz), U.S. Pat. No. 4,844,406 (Wilson), U.S. Pat. No. 4,860,826 (Land), U.S. Pat. No. 5,743,332 (Lam), U.S. Pat. No. 5,765,813 (Lam), U.S. Pat. No. 5,875,841 (Wright), U.S. Pat. No. 6,079,489 (Hutt), U.S. Pat. No. 6,152,231 (Grenke), U.S. Pat. No. 6,557,643 (Hall), U.S. Pat. No. 6,595,278 (Lam), U.S. Pat. No. 7,673,674 (Lam), U.S. Pat. No. 7,000,888 (Wright), U.S. Pat. No. 7,806,665 (Mello), and U.S. Pat. No. 8,132,618 (Blaquiere).
However, there is a continuing need for improvement in devices for reducing backspin in rod strings used in progressive cavity pumps.